Activating mutations in the c-kit protooncogene codon 816 are found in most patients with systemic mastocytosis involving the bone marrow. Since c-kit encodes a transmembrane receptor tyrosine kinase, inhibition of this pathway by drugs such as imatinib (STI-571; Gleevec) has been evaluated as a potential therapeutic strategy in mast cell disease. Using primary mast cells obtained from patient bone marrow aspirates, Gleevec has been shown to exert a potent cytotoxic activity on mast cells carrying c-kit with a wild-type codon 816. On the other hand, the drug was relatively ineffective in killing mast cells carrying a codon 816 c-kit mutation. To complement these observations, a patient with a previously undescribed variant of mastocytosis was found to have a novel activating mutation in the transmembrane portion of c-kit, and showed a dramatic response to imatinib. Eosinophils represent a hematopoietic lineage which is involved heavily in the pathogenesis of some allergic diseases such as asthma and drug reactions. Their numbers are pathologically increased in idiopathic hypereosinophilic syndrome (HES), a disease with protean presentations similar to mastocytosis. One variant of HES with myeloproliferative features (MP-HES) has been observed to be associated with elevated plasma tryptase levels and abnormally shaped mast cells. In collaboration with LPD, we thus investigated the pathologic features of mast cells in MP-HES. We found a unique mast cell phenotype with aberrant surface expression of CD25 and lack of CD2. C-kit codon 816 mutations, generally detected in mastocytosis, were absent in MP-HES mast cells. Patients with this disease variant had the recently described fusion of the Fip1-like 1 gene to the platelet-derived growth factor receptor alpha gene. Most importantly, all patients with HES and aberrant mast cells treated with imatinib demonstrated a clinical and hematologic response in this otherwise incurable disease. This study showed that that determination of serum tryptase levels and mast cell phenotype appears to be a sensitive marker of a myeloproliferative variant of MP-HES and helps to identify patients who may respond to therapy with imatinib. In collaboration with the TCBS, NCI, we have also investigated the effects of 17-AAG on mast cells carrying wild type and mutated c-kit. 17-AAG is a benzoquinoid ansamycin antibiotic, which binds to heat shock protein 90 (hsp90), and causes destabilization of various hsp90-dependent kinases important in oncogenesis. We showed that treatment of the HMC-1 cell line harboring the D816V c-kit mutation with 17-AAG caused both the level and activity of KIT to be down-regulated as early as 2 hours following drug exposure. These data were validated using Cos-7 cells transfected with wild type and mutated c-kit. In addition, neoplastic mast cells isolated from patients with mastocytosis and incubated with 17-AAG ex vivo were selectively sensitive to the drug, compared to the mononuclear fraction as a whole. These data provide compelling evidence that 17-AAG may be effective in the treatment of mastocytosis, as well as other c-kit related diseases including gastrointestinal stromal tumors, and some sub-types of acute myelogenous leukemia and germ-cell tumors. We are currently in the process of writing a clinical protocol to test the effects of 17-AAG in mastocytosis, in collaboration with investigators from NCI. In order to help interpretation of these future clinical trials, standardized response criteria were formulated in collaboration with other researchers in the field.